Electrodialysis is based on the phenomenon that in aqueous electrolyte solution conventional ion-exchange membranes are permselective in that they are capable of selectively transporting charged species across their boundaries under the influence of an electric field. Cation-exchange membranes carry fixed negatively charged groups and, therefore, pass preferentially positively charged ions, and anion-exchange membranes carry fixed positively charged groups and pass preferentially negatively charged ions. Ideally, permselective ion-exchange membranes allow the passage of ions of one sign and completely prevent the passage of the ions of the opposite sign. In practice, however, permselectivity is as a rule not ideal. For a given membrane, it is highest in dilute solutions and decreases with increasing salt concentration.
Bipolar (BP) membranes comprise a cation-exchange (C) and anion-exchange (A) layer. The BP membranes of different compositions and their applications are described in several articles and patents. For example, U.S. Pat. No. 3,372,101 (Kollsman) describes the preparation of such membranes by bonding together separately prepared anion- and cation-exchange films or membranes, using a hydraulic press.
U.S. Pat. Nos. 4,024,043 and 4,057,481 (both to Dege et al.) describe single-film bipolar membranes. The film contains a high amount of an insoluble cross-linked aromatic polymer, to which, in a first step, highly dissociable cationic exchange groups are chemically bonded at one side and highly dissociable anionic exchange groups are then chemically bonded to the other side. U.S. Pat. No. 3,562,139 (Leitz) describes asymmetrical BP membranes prepared in a similar way. In these asymmetrical membranes, the two layers of opposite polarity have different charge densities which lead to different permselectivities and to different concentration dependencies of the latter. In such BP membranes, the thickness of the A- and C-layers may be different. Japanese Patent Publications Nos. 78-158638 and 79-7196, (both of Tokuyama Soda Co. Ltd.) disclose a similar method with the added feature of masking one side of the film while reacting the other. In a recent patent publication, the successful joining of separately prepared A-and C-membranes was reported (PCT/AU88/00279). Another approach to the preparation of BP membranes involve casting two layers one on top of the other. Such an approach was reported by Bauer et al. in Desalination 68,279 (1988). In this example, Bauer et al. cast an A-layer made of chloromethylated polystyrene, mixed with other polymers which were then aminated, followed by casting the C-layer on top of the A-layer.
In the following, an electrodialysis set-up in which the A-layer of a BP membrane faces the anode will be referred to as the bipolar direction while a set-up in which the C-layer faces the anode will be referred to as the reverse direction.
In the performance of electrodialysis with a BP membrane in the bipolar direction, ions are drawn out from the interface between the membranes of opposite polarity-anions through the A-layer and cations through the C-layer. This effectively exhausts the salt ions from the interface, and in consequence of the processes that occur at the interface between the A- and C-layers, the overall result is a continuous dissociation of water into H.sup.+ and OH.sup.- ions with the generation of equivalent amounts of acid and base. Such a process is often referred to in the literature as "water splitting". However, to avoid confusion with the process of water decomposition into hydrogen and oxygen gases, the process will be referred to herein as "acid-base generation".
When a BP membrane operates in the reverse direction, salt is pushed into the membrane interface from both surfaces. This salt flow causes water flow into the membrane both by osmosis and by electro-osmosis, which tends to destroy the BP membrane by blister formation or separation of the two layers. In consequence, all known BP membranes are sensitive to current reversal and have a short operational life under these conditions which is a major shortcoming of EDR processes in which the current is reversed periodically, say every 15 to 20 minutes, to avoid membrane fouling. There has, thus, been a need in the art of EDR processes for BP membranes which are not sensitive to current reversal.
It is, thus, the object of the present invention to fulfil this long-felt need and provide improved BP membranes of high mechanical stability suitable for use in EDR processes, which are not prone to blister formation and are capable of withstanding the strains of periodic current reversal during long periods of time.